#include <stdio.h>
int main(void)
{
const float max_val = 4096;
float test = ((4024 / max_val) * 1.8);
printf("%f\n", test);
return 0;
}
$ gcc test.c -Wall -o test
$ ./test
1.768359
On Mon, Nov 2, 2015 at 8:59 AM, William Hermans <[email protected]> wrote:
> Anyway, since you do not seem to know why that does not work.
>
> Math on two integer values can not give out a float value. This is not a
> math problem, but a compiler behavior issue. Intentional or no, one value
> in the equation brackets *has* to be forced to a float value as such.
>
> 1) ((4024 / 4096.0) * 1.8)
> *OR*
> 2) const float max_value = 4096; ((4024 / max_value) * 1.8)
>
> It's also why I suggested using a const above.
>
> On Mon, Nov 2, 2015 at 8:16 AM, William Hermans <[email protected]> wrote:
>
>> really now. Prey tell, enlighten us then with your logic.
>>
>> On Mon, Nov 2, 2015 at 5:10 AM, Chad Baker <[email protected]> wrote:
>>
>>> No, I don't, but having taught programming for over 40 years, I know
>>> that students would take your equation and plug it in and then be surprised
>>> that they get 0 for an answer.
>>>
>>> On 11/1/2015 6:13 PM, William Hermans wrote:
>>>
>>> *Or 4024 * 1.8 / 4096?*
>>>>
>>>
>>> Ok, so let us put this another way. Do you think that is clear when
>>> written in code ? What if 1.8 were actually assigned to a constant
>>> pointer ?
>>>
>>>
>>>
>>> On Sat, Oct 31, 2015 at 5:01 AM, Chad Baker <[email protected]> wrote:
>>>
>>>> Or 4024 * 1.8 / 4096?
>>>>
>>>>
>>>> On 10/30/2015 12:18 PM, William Hermans wrote:
>>>>
>>>> By the way, the ADC gives out a value of 0-4095. So scaling voltage is
>>>> really simple. Say you get a value of 4024 for a reading. In order to get a
>>>> reading from 0-1.8v from this . . .
>>>>
>>>> ((4024 / 4096) * 1.8) or 1.768359375v
>>>>
>>>> The max voltage on the pins still can not exceed 1.8v, this is why one
>>>> needs an opamp, or similar device to keep the input scaled down. But, in
>>>> this manner you could even track large voltages. Hundreds or even thousands
>>>> of volts, at the cost of resolution. Which unfortunately can not be
>>>> avoided. So going by the above example . . .
>>>>
>>>> ((4024 / 4096) * 300) or 294.7265625v
>>>>
>>>> Does this make sense ?
>>>>
>>>>
>>>> On Fri, Oct 30, 2015 at 9:59 AM, William Hermans < <[email protected]>
>>>> [email protected]> wrote:
>>>>
>>>>> *BBB and Linux are new to me. One of the things I would like to do is
>>>>>> compute a ratio between two voltages, each of which might exceed 1.8 VDC.
>>>>>> One solution might be to use DACs to sent feedback voltages to
>>>>>> operational
>>>>>> amplifiers that are supplied by 1.8 VDC. The goal is to determine the
>>>>>> ratio as quickly as possible and without waiting for Linux to get around
>>>>>> to
>>>>>> it (waiting for the ADC to happen).*
>>>>>>
>>>>>
>>>>>
>>>>> What sample rate do you need ? If something around only a few thousand
>>>>> samples a second. This can be done from userspace without using the PRU's.
>>>>> You can even use mmap() with /dev/mem/ and directly manipulate the ADC's
>>>>> registers, which can sample the ADC faster than 200ksps. However, the
>>>>> sample rate would not be as deterministic as using the PRU's.
>>>>>
>>>>> You can use a PRU to read the ADC's for sure. If you search the web
>>>>> for Beaglebone PRU + ADC
>>>>> , you will find many ways. People have been doing this for a long
>>>>> time( since the beaglebone white ), and if you search this google group,
>>>>> you'll even find full code for something someone called "ADCcollector".
>>>>> It's C, and PASM assembly, but is a very short / easy read /
>>>>> understanding.
>>>>>
>>>>> As for how to physically connect to the ADC, I'd listen to
>>>>> evilwulfile. He's been an EE for longer than many people on this group
>>>>> have
>>>>> been alive, and he knows his stuff ( I know him personally ). Don't risk
>>>>> burning up your board, as I've seen several people on these groups do . .
>>>>> .
>>>>>
>>>>> On Fri, Oct 30, 2015 at 9:43 AM, TJF < <[email protected]>
>>>>> [email protected]> wrote:
>>>>>
>>>>>> Hi John!
>>>>>>
>>>>>> As Przemek mentioned, a simple voltage divider can scale the input
>>>>>> voltage. Additionaly I use a LED (orange color) between the ADC input and
>>>>>> ground in order to block overvoltage. (The LED starts to light up at 1V5
>>>>>> and consumes full power at 1V8. 1 / 6 of the ADC resolution get lost (is
>>>>>> less accurate), but the burning LED as indicatior is a nice side effect.)
>>>>>>
>>>>>> Regarding software you should have a look at libpruio
>>>>>> <http://hackaday.com/2015/02/16/library-upgrade-to-pru-gives-fast-io-on-beaglebone/#comments>
>>>>>> .
>>>>>>
>>>>>> BR
>>>>>> --
>>>>>> For more options, visit <http://beagleboard.org/discuss>
>>>>>> http://beagleboard.org/discuss
>>>>>> ---
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>>>>>> [email protected].
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>>>>>> https://groups.google.com/d/optout.
>>>>>>
>>>>>
>>>>>
>>>> --
>>>> For more options, visit http://beagleboard.org/discuss
>>>> ---
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>>>>
>>>>
>>>>
>>>> --
>>>> Chad Baker Memphis, TN
>>>> --
>>>> For more options, visit http://beagleboard.org/discuss
>>>> ---
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>>>>
>>>
>>> --
>>> For more options, visit http://beagleboard.org/discuss
>>> ---
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>>>
>>>
>>> --
>>> Chad Baker Memphis, TN
>>>
>>> --
>>> For more options, visit http://beagleboard.org/discuss
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>>>
>>
>>
>
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